This invention relates to the sensing of tension in such tissue as ligaments, tendons, muscle, and skin.
There are few common elective orthopedic procedures that have as many marginal results and complications as knee ligament surgery. The goal of knee surgery is to restore the normal kinematics of the knee. There are no universal protocols for ligament fixation. Setting a proper tension in a repaired or grafted ligament is important in the success of knee repair and the return of the patient to pre-injury activity levels. To regain normal anteroposterior translation, proper tension needs to be applied on the graft. If the tension is too low, the joint will be wobbly. If the tension is too high, the range of motion of the joint will be restricted, resulting in abnormal stress on the articular cartilage and the menisci. Excessive tension also interferes with the revascularization of the graft.
During knee surgery, the position of the knee and the load applied to the graft are dependent on the preference and experience of the surgeon. It has been suggested that graft fixation with a full extension may over-constrain the knee and that fixation at flexion and an anterior tibial load would best restore knee biomechanics. Applying graft tension at extension and checking the tension at 20° of flexion seems to be the norm. The strain value of the ligament or tendon is a macroscopic measure of the deformation. It is computed by dividing the change in length of the ligament or tendon by the unstressed length and is usually expressed as a percentage. Peak strain measured in vivo in the human anterior cruciate ligament (ACL) is about 4.4% (B. D. Beynnon and B. C. Fleming, Anterior cruciate ligament strain in-vivo: A review of previous work, Journal of Biomechanics, 31, 519-525, 1998). A high initial ACL tension (up to 80 N) may reduce the postoperative anterior laxity of the knee.
Ligaments consist of densely packed collagen fiber bundles arrayed in parallel along the length of the tissue. There are varying amounts of folding or crimp in the collagen fibrils, allowing for increasing resistance to increasing loads. Recruitment of additional fibrils occurs with increasing deformation under load. As the number of load-bearing fibrils increases, an increase in tissue stiffness results. The two major functions of the knee ligaments are to provide dynamic guide for knee motion and mechanical restraint to prevent abnormal translations. Knee instability may result in giving way under stress, re-injury of the knee, and early degenerative arthritis. The ultimate load of about 1725±269 N and the stiffness of about 182±33 N/mm are considered the gold standards.
The ACL is the most frequently injured ligament. It is composed of fascicular subunits within larger functional bands. The bands are selectively recruited during tensile loading. Fiber recruitment is due to the specific location of the insertions of the ACL on the tibia and the femur as different fibers attach to different locations on each bone. The fibers change length by a straightening of the crimp. The core of the ACL is the tension-carrying fibrous collagen. The ACL contains viscoelastic elements, blood vessels, nerves, and fibroblasts. The choice of graft for the replacement of ACL is controversial; prosthetic ligaments appear to result in more complications than autografts. Since strength is a major consideration in the selection of the graft, the two most common grafts are the central third of the patellar ligament and the hamstring tendon. For the first two months after implantation, the main factor affecting the structural strength of either graft is not the load-bearing capacity of the tissue but the point of fixation of the graft to the bone. The tendon tissues seem to lose some strength during the early healing period. Proper placement of the tunnels in the femur and the tibia during ACL reconstruction is important in minimizing permanent stretching of the graft.
The objective of tensioning the graft is to establish and maintain normal stability of the joint by eliminating wobble and restoring movement to the normal range. It has been shown that the initial forces in a graft are greatest near extension when tension is applied to the graft from its proximal end with the knee at 30° of flexion, and the forces in the graft may decrease by as much as 30% soon after fixation unless the graft has been cyclically preconditioned.
Ligaments and tendons function over a relatively small range of strain, typically less than 8% of its unstressed length. Joints have at least two ligaments that work opposed to each other to keep motion in the normal range. The human body has more than 1,000 ligaments and tendons. These tissues control the kinetic and kinematic actions of joints. The stress-strain relation of these tissues allows bones to move smoothly under low stress and limits the motion of the bones under high stress. Ligaments stabilize joints and guide them through smooth motions. Tendons transmit the dynamic forces generated by muscles across joints.
Ligaments contain water, elastin, proteoglycans, and packed collagen fibers that run parallel to the longitudinal axis of the ligament. The proteoglycans and water provide lubrication and spacing needed for the gliding function of joints. Collagen fibers in ligaments are arranged in varying degrees of crimp such that an increase in tensile force, a force directed along the axis of the ligament, results in the recruitment of more fibers to resist the load.
Ligaments and tendons are ordered structures. The collagen fibers lie parallel to the ligament (or tendon) axis displaying a cylindrical axis of symmetry. The motion of the atoms comprising any material matter such as collagen can be characterized by a collection of fundamental modes of vibration. Depending on the symmetry of the molecule, some normal modes of vibration may interact with optical radiation. One takes advantage of these optically active modes of vibration to characterize properties of the molecule. These vibrational modes can be investigated using either absorption spectroscopy method if the vibration has a permanent dipole moment and/or Raman spectroscopy if the vibration results in a change in “differential polarizability,” i.e., the change in polarizability due to the motion of atoms involved in the vibration.
When stress is applied to the ligament, it is distributed through the entire structure of the ligament. The effect of the stress on vibrations along the direction of the stress is different from that on the vibrations whose motion is normal to the applied stress. In a very simplified model, one can think of the vibration as a simple oscillator governed by an effective mass and a force constant. The effect of the stress is to change the value of the force constant. Changes in the frequencies of the vibrations whose motion is along the axis of symmetry of the ligament or tendon are expected when the ligament or tendon is under stress. Due to the low frequency, between 500 cm−1 and 2000 cm−1, of the vibrational modes, the absorption spectrum of the ligament lies between 5 and 20 μm, a region where optical fibers are not readily available.
Needs exist for improved methods and devices for determining proper tensions in ligaments, tendons and other tissues.